Propagation Of Evanescent Waves Research Articles

A lightweight concrete composite material with improved EMI shielding by using a chiral particle array

This paper proposes a structure for concrete composite materials for electromagnetic interference shielding applications. It comprises an array of helical-shaped conductive particles as chiral additives. Unlike the previous studies using large quantities of conductive additives, this study provides a lightweight concrete composite due to the utilization of a small number of additives with a high level of shielding effectiveness in a wide frequency range. The heart of the proposed structure lies in leveraging the evanescent wave propagation in a circular waveguide, resulting in considerable shielding effectiveness. Under particular conditions, the helical particles can imitate a below-cutoff cylindrical waveguide and its dominant mode surface current on the helical wire. This phenomenon significantly attenuates the transmitted power from the array of helical particles in its resonance frequency range. Besides presenting the composition that exploits the magnetoelectric properties of the particles, this paper compares it with a traditional concrete composite, including randomly distributed sinusoidal steel rods. This second approach is examined using both experimental measurement and full-wave simulation methods. The results of this study indicate that the appropriate geometry of the conductive additives, in this case, chiral particles, and their arrangement in a regular array rather than a random distribution can enhance the efficiency of the conductive additives. This idea paves the way for more robust, efficient, and lightweight concrete composite materials, thanks to the recent advances in modern civil engineering manufacturing methods.

Critically synchronized brain waves form an effective, robust and flexible basis for human memory and learning

The effectiveness, robustness, and flexibility of memory and learning constitute the very essence of human natural intelligence, cognition, and consciousness. However, currently accepted views on these subjects have, to date, been put forth without any basis on a true physical theory of how the brain communicates internally via its electrical signals. This lack of a solid theoretical framework has implications not only for our understanding of how the brain works, but also for wide range of computational models developed from the standard orthodox view of brain neuronal organization and brain network derived functioning based on the Hodgkin–Huxley ad-hoc circuit analogies that have produced a multitude of Artificial, Recurrent, Convolution, Spiking, etc., Neural Networks (ARCSe NNs) that have in turn led to the standard algorithms that form the basis of artificial intelligence (AI) and machine learning (ML) methods. Our hypothesis, based upon our recently developed physical model of weakly evanescent brain wave propagation (WETCOW) is that, contrary to the current orthodox model that brain neurons just integrate and fire under accompaniment of slow leaking, they can instead perform much more sophisticated tasks of efficient coherent synchronization/desynchronization guided by the collective influence of propagating nonlinear near critical brain waves, the waves that currently assumed to be nothing but inconsequential subthreshold noise. In this paper we highlight the learning and memory capabilities of our WETCOW framework and then apply it to the specific application of AI/ML and Neural Networks. We demonstrate that the learning inspired by these critically synchronized brain waves is shallow, yet its timing and accuracy outperforms deep ARCSe counterparts on standard test datasets. These results have implications for both our understanding of brain function and for the wide range of AI/ML applications.

Complex band structure and evanescent wave propagation of composite corrugated phononic crystal beams

Complex band structure and evanescent wave propagation of composite corrugated phononic crystal beams

Evanescent waves in hybrid poroelastic metamaterials with interface effects

The propagation of evanescent waves in hybrid poroelastic metamaterials is investigated by considering interface effects. Hybrid metamaterials consist of a single-phased (acoustic or elastic) medium and a poroelastic medium. To establish the finite element model of elastic/poroelastic and fluid/poroelastic interfaces, weak integral forms of wave equations for elastic, fluid, and poroelastic media and boundary conditions at the interfaces between different media are first given. Next, the expressions for displacement and pressure in the frame of Bloch’s theorem are substituted into the dynamical equations to obtain general forms suitable for periodic metamaterials, from which the complex band structure and the frequency response of hybrid metamaterials are calculated. The influence of geometrical and material parameters, as well as the viscosity of the pore fluid on the propagation of elastic waves are discussed. The results and discussions show that flat bands and narrow locally resonant band gaps appear for elastic/poroelastic metamaterials. A quasi-resonance Bragg band gap is formed in the case of an elastic inclusion in a poroelastic matrix. Furthermore, a transition from an avoided crossing to a wave-number band gap is obtained by adjusting the geometrical and material parameters of the elastic inclusion. For the case of fluid inclusion in a poroelastic matrix with the open-pore interface, a cut-off frequency for the fast longitudinal wave is observed. However, only an avoided crossing is produced for the sealed-pore interface. For both cases, the phase velocity of the shear vertical (SV) wave decreases faster than that of the slow longitudinal wave as the radius of the inclusion increases. When the viscosity of the pore fluid is considered for elastic inclusion in a poroelastic matrix, the transmission dip at the ‘quasi-resonance Bragg’ band gap disappears. However, the transmission dip in the locally resonant band gap of the SV wave becomes slightly shallower and smoother than in the inviscid case. This study is relevant to practical applications of hybrid single-phased and poroelastic metamaterials, e.g., for coastal engineering and civil engineering.

Stabilization of Evanescent Wave Propagation Operators

Stabilization of Evanescent Wave Propagation Operators

Propagation of visible light in nanostructured niobium stripes embedded in a dielectric polymer

Abstract Nanometric metallic stripes allow the transmission of optical signals via the excitation and propagation of surface-localized evanescent electromagnetic waves, with important applications in the field of nano-photonics. Whereas this kind of plasmonic phenomena typically exploits noble metals, like Ag or Au, other materials can exhibit viable light-transport efficiency. In this work, we demonstrate the transport of visible light in nanometric niobium stripes coupled with a dielectric polymeric layer, exploiting the remotely-excited/detected Raman signal of black phosphorus (bP) as the probe. The light-transport mechanism is ascribed to the generation of surface plasmon polaritons at the Nb/polymer interface. The propagation length is limited due to the lossy nature of niobium in the optical range, but this material may allow the exploitation of specific functionalities that are absent in noble-metal counterparts.

Deep-learning-based inverse design of phononic crystals for anticipated wave attenuation

Bandgaps of phononic crystals dominating the propagation of evanescent waves have received significant attention recently, which can be determined and tuned by the topology of a unit cell. Predicting a band structure and designing topological structures with desirable characteristics have become a research hotspot. In this study, a data-driven deep learning framework is applied to arrive at the prediction of the band structure and the inverse design of topology. A convolutional neural network is trained to predict band structures of phononic crystals. After training a generative adversarial network, the generator is concatenated with the convolutional neural network for inverse design. Meanwhile, a complex band structure of phononic crystals is computed by the periodic spectral finite element method to present the spatial decay of evanescent waves. The topology with the greater spatial attenuation is screened from the ground truth topology and the inversely designed topology. Finally, an optimized topological phononic crystal with an anticipated bandgap is obtained, which has the potential for better acoustic insulation and vibration isolation.

Propagation of evanescent wave through surface-attached nanobubbles: A 2D simulation

We numerically study the propagation of evanescent waves at the interface between water and glass. Due to the existence of surface-attached nanobubbles, the intensity of the evanescent wave passed into water increases by 30 times, which can be used in the dark-field inspection. The relationship between the intensity of the evanescent wave and contact angle is found and analyzed. It provides a possible method to measure the contact angle. Additionally, the theoretical distribution of bright and dark areas on nanobubbles is consistent with experimental data, where the middle of the nanobubble is darker than the edge. The right edge of the bubble is brighter than the left edge, which corresponds to the incident direction of the light source.

Complex band structure and evanescent Bloch wave propagation of periodic nested acoustic black hole phononic structure

In this study, the periodic nested acoustic black hole phononic structure is presented, and different complex band structures and corresponding evanescent Bloch wave propagation are discussed. Vibration transmissions and modes indicated that the corresponding regions of the bandgaps were due to the coupling effect between the Fano-like resonance and Bragg scattering, which also presented the negative mass and stiffness characteristics. Moreover, strain tensors at interface and central sections of the unit cells were used to evaluate the structural instability and failure. Contrast models proved the superiority of the periodic nested acoustic black hole phononic structure for evanescent Bloch wave control and vibration experiments verified its effectiveness. The relevant conclusions can be used in the vibration reduction design of lightweight structure and enrich the relevant research experience on acoustic black hole.

Focusing a Two-Dimensional Acoustic Vortex Beyond Diffraction Limit on an Ultrathin Structured Surface

We propose and experimentally demonstrate a mechanism for focusing two-dimensional (2D) vortex for airborne sound beyond diffraction limit on an ultrathin structured surface in free space. A simple design of unit cell is presented as a practical implementation, which is capable of exciting evanescent wave and modulating its propagation phase over full 0-to-2\ensuremath{\pi} range. By analytically deriving the dispersion relationship and desired azimuthal distribution of effective parameters, we elucidate how to guide the propagation of excited evanescent waves in the vicinity of an unbounded surface decorated with designed structures and focus such surface vortex within a target region despite its compactness and nonaxisymmetry. The effectiveness of our mechanism is demonstrated numerically and experimentally via production and confinement of 2D vortex on a square surface at subwavelength spatial resolution. We anticipate our methodology with no need of a 2D system, bulky device size, geometric symmetry, active elements, or complicated phased array to offer new possibilities for the miniaturization and integration of planar vortex devices and may promote their on-chip applications in various fields such as by significantly boosting the information density and manipulation precision.

Sensitivity enhancement of super resolution breast tumour imaging with far-field time reversal mirror integrating with multi-layered sub-wavelength patch scatterers

We present numerical simulation study of super-resolution imaging of closely spaced sub-wavelength size breast tumours with low cost microwave MIMO bowtie antenna array integrating with near-field planar multi-layered sub-wavelength patch (MLSWP) scatterers by using a monopole as the excitation source. The MLSWP scatterers, serving as transforming structures for the propagation of evanescent waves for sub-wavelength breast tumours imaging with far-field time reversal technique incorporating with bowtie antenna time reversal mirror, are investigated in this paper. The use of the MLSWP scatterers has enhanced the directivity and gain of the traditional monopole resulting in the enhanced scattering amplitudes of the tumour inclusions in the breast. The scattering intensity of the tumours has been enhanced as compared to imaging without the MLSWP scatterers. We propose a tumour imaging scheme using a bowtie antenna array with far-field electromagnetic time reversal for which tumours of sub-wavelength dimensions of 2 mm, 4 mm and 6 mm at sub-wavelength inter tumour distances have been imaged from our simple breast model with super-resolution operating at 2.45 GHz. This will greatly enhance the early imaging of breast tumours in the fight against breast cancer whiles attaining a cost effective biomedical imaging system since the antennas and MLSWP scatterers are low cost microwave components.

Quantitative Analysis of Photon Density of States for One-Dimensional Photonic Crystals in a Rectangular Waveguide

Light propagation in one-dimensional (1D) photonic crystals (PCs) enclosed in a rectangular waveguide is investigated in order to achieve a complete photonic band gap (PBG) while avoiding the difficulty in fabricating 3D PCs. This work complements our two previous articles (Phys. Rev. E) that quantitatively analyzed omnidirectional light propagation in 1D and 2D PCs, respectively, both showing that a complete PBG cannot exist if an evanescent wave propagation is involved. Here, we present a quantitative analysis of the transmission functions, the band structures, and the photon density of states (PDOS) for both the transverse electric (TE) and transverse magnetic (TM) polarization modes of the periodic multilayer heterostructure confined in a rectangular waveguide. The PDOS of the quasi-1D photonic crystal for both the TE and TM modes are obtained, respectively. It is demonstrated that a “complete PBG” can be obtained for some frequency ranges and categorized into three types: (1) below the cutoff frequency of the fundamental TE mode, (2) within the PBG of the fundamental TE mode but below the cutoff frequency of the next higher order mode, and (3) within an overlap of the PBGs of either TE modes, TM modes, or both. These results are of general importance and relevance to the dipole radiation or spontaneous emission by an atom in quasi-1D periodic structures and may have applications in future photonic quantum technologies.

Precise control of evanescent scattering by self-assembled ferromagnetic particles for optical sensing with tunable sensitivity.

We propose an optical sensing platform that uses evanescent scattering through precise manipulation of self-assembled ferromagnetic particle columns. The movement of the column tips can be controlled dynamically down to a submicron range by an external actuation, namely, a magnetic field, for interacting with evanescent wave propagation along an optical waveguide that causes a change in its output intensity for optical sensing. To demonstrate the idea, an AC current sensor with only a 5mm interaction length is proposed and realized. Furthermore, its sensitivity is tunable within 9-20dB/A by varying a DC-biased signal. The platform shows favorable signal reversibility, stability, broadband operation, and real-time response.

Evaluation of thin discontinuities in planar conducting materials using the diffraction of electromagnetic field

The current stage of nondestructive evaluation techniques imposes the development of new electromagnetic (EM) methods that are based on high spatial resolution and increased sensitivity. In order to achieve high performance, the work frequencies must be either radifrequencies or microwaves. At these frequencies, at the dielectric/conductor interface, plasmon polaritons can appear, propagating between conductive regions as evanescent waves. In order to use the evanescent wave that can appear even if the slits width is much smaller that the wavwelength of incident EM wave, a sensor with metamaterial (MM) is used. The study of the EM field diffraction against the edge of long thin discontinuity placed under the inspected surface of a conductive plate has been performed using the geometrical optics principles. This type of sensor having the reception coils shielded by a conductive screen with a circular aperture placed in the front of reception coil of emission reception sensor has been developed and “transported” information for obtaining of magnified image of the conductive structures inspected. This work presents a sensor, using MM conical Swiss roll type that allows the propagation of evanescent waves and the electromagnetic images are magnified. The test method can be successfully applied in a variety of applications of maxim importance such as defect/damage detection in materials used in automotive and aviation technologies. Applying this testing method, spatial resolution can be improved.

High-order finite elements for the solution of Helmholtz problems

In this paper, two high-order finite element models are investigated for the solution of two-dimensional wave problems governed by the Helmholtz equation. Plane wave enriched finite elements, developed in the Partition of Unity Finite Element Method (PUFEM), and high-order Lagrangian-polynomial based finite elements are considered. In the latter model, the Chebyshev-Gauss-Lobatto nodal distribution is adopted and the approach is often referred to as the Spectral Element Method (SEM). The two strategies, PUFEM and SEM, were developed separately and the current study provides data on how they compare for solving short wave problems, in which the characteristic dimension is a multiple of the wavelength. The considered test examples include wave scattering by a rigid circular cylinder, evanescent wave cases and propagation of waves in a duct with rigid walls. The two approaches are assessed in terms of accuracy for increasing SEM order and PUFEM enrichment. The conditioning, discretization level, total number of storage locations and total number of non-zero entries are also compared.

Study on coherent coupling of multi-core PCFs

In this work, we used R-Soft software to simulate the coupling between multi-core photonic crystal fibers (PCFs) with different core diameters and distances. The simulation results show that the core distance within two wavelengths, different core diameters between the mode fields can be effectively coupled. The results are consistent with the evanescent wave propagation length of calculation. According to the designed structure based on the high temperature melting method, the multi-core PCF can be obtained by using two times technique of the optical fiber drawing tower. The core production of (2-, 7-, 19-core PCF) of coherent synthesis experiment, observed apparent interference fringes in the experiment, which shows that the mode fields of the multiple cores meet the coherent conditions. This article provides a theoretical basis and guidance to effectively achieve a coherent coupling for the further design and preparation of multi-core fibers (more than 19 cores).

Maximizing spatial decay of evanescent waves in phononic crystals by topology optimization

The propagation of evanescent waves inside phononic band gaps is important for the design of phononic crystals with desirable functionalities. This paper extends the bi-directional evolutionary structure optimization (BESO) method to the design of phononic crystals for maximizing spatial decay of evanescent waves. The optimization objective is to enlarge the minimum imaginary part of wave vectors at a specified frequency. The study is systematically conducted for both out-of-plane and in-plane waves at various frequencies. Numerical examples demonstrate that the proposed optimization algorithm is effective for designing phononic crystals with maximum spatial decay of evanescent waves. Various topological patterns of optimized phononic crystals are given. The results also show that the proposed optimization algorithm can successfully overcome difficulties in opening band gap at desirable frequencies, especially for the complete band gap and multiple band gaps.

Linear MHD Wave Propagation in Time-Dependent Flux Tube

In this article, we evaluate the time-dependent wave properties and the damping rate of propagating fast magneto-hydrodynamic (MHD) waves when energy leakage into a magnetised atmosphere is considered. By considering a cold plasma, initial investigations into the evolution of MHD wave damping through this energy leakage will take place. The time-dependent governing equations have been derived previously in Williamson and Erdelyi (2014a, Solar Phys.289, 899 – 909) and are now solved when the assumption of evanescent wave propagation in the outside of the waveguide is relaxed. The dispersion relation for leaky waves applicable to a straight magnetic field is determined in both an arbitrary tube and a thin-tube approximation. By analytically solving the dispersion relation in the thin-tube approximation, the explicit expressions for the temporal evolution of the dynamic frequency and wavenumber are determined. The damping rate is, then, obtained from the dispersion relation and is shown to decrease as the density ratio increases. By comparing the decrease in damping rate to the increase in damping for a stationary system, as shown, we aim to point out that energy leakage may not be as efficient a damping mechanism as previously thought.

Visualization of Acoustic Evanescent Waves by the Stroboscopic Photoelastic Method

It is well known that evanescent waves are produced when an incident wave strikes an interface at an angle exceeding a critical angle and that they exhibit exponential decay within the refractive medium. Evanescent waves have been extensively studied and have attracted substantial attention for application in technology allowing expansion into the nano scale. However, the propagation of evanescent waves is not well understood visually. We have achieved acoustic evanescent waves produced when a propagating incident wave impinges on a water/glass interface at a post-critical angle, using the Fresnel method in the water and the photoelastic method in the glass.

Graphene-based tunable broadband hyperlens for far-field subdiffraction imaging at mid-infrared frequencies

Considering the dielectric permittivity of graphene can be tuned to be negative by external electric field, we propose to construct alternating graphene/dielectric multilayer based optical hyperlens for far-field subdiffraction imaging at mid-infrared frequencies. For such a scheme, hyperbolic dispersion curve can be achieved under the condition that the thickness of dielectric layer is made comparable to that of graphene layer, which is capable of supporting the propagation of evanescent wave with large wave vector. Simulation results by finite-element method demonstrate that two point sources with separation far below the diffraction limit can be magnified by the systems to the extent that conventional far-field optical microscopy can further manipulate. Such a hyperlens has the advantage of operating in a wideband region due to the tunability of graphene's dielectric permittivity as opposed to previous metal based hyperlens, enabling the potential applications in real-time super-resolution imaging, nanolithography, and sensing.